1. Field of the Invention
The present invention relates to a hot roller type thermal fixation device to be incorporated in an electrostatic recording apparatus, particularly to a hot roller type thermal fixation device suitable for thermally fixing a multicolor toner image.
2. Description of the Related Art
Generally speaking, in an electrostatic recording apparatus, the following processes are sequentially carried out in the recording operation; a latent-image-forming process for forming an electrostatic latent image on an electrostatic image carrier such as a photo-sensitive body or a dielectric body, a developing process for electrostatically developing the electrostatic latent image by a charged toner so that a charged toner image is obtained, a transfer process for electrostatically transferring the charged toner image onto a recording medium such as a recording paper, and a fixing process for fixing the transferred image on the recording paper. When a multicolor recording is carried out in such an electrostatic recording apparatus, the latent-image-forming process, the developing process and the transfer process are repeated at least twice. During each the repetition, a charged toner image is formed with one color toner in the developing process, which is transferred onto the same recording paper to be superimposed with a different color toner image in the transfer process. That is, at least two toner images are superimposed with each other on the recording paper to form a multicolor toner image. Thereafter, the recording paper sends to the fixing process wherein the multicolor toner image consisting of different colors is fixed onto the recording paper. As is well-known, when the full-color recording is carried out, four color toners are used; a yellow toner, a cyan toner, a magenta toner and a black toner.
As typical fixation device, a well-known hot roller type thermal fixation device, wherein the fixation of toner image onto the recording paper is carried out by thermally melting the toner image under pressure is used. Specifically, the hot roller type thermal fixation device includes a heated roller and a backup roller which is in press-contact with the heated roller, wherein the heated roller is generally constituted by a cylindrical member formed of a suitable material such as aluminum or stainless steel and a heater body disposed centrally in the cylindrical member, while, the backup roller is also formed of a metallic cylindrical member. The recording paper passes through a nip between the heated roller and the backup roller while allowing the toner image thereon to be in contact with the heated roller surface, to which the toner image is pressed and is melted to be fixed on the recording medium. In this regard, the backup roller may also have a heater body therein if necessary. Other fixation devices have been known, such as a flash type using a xenon lamp or an oven type. Different from the above-mentioned heated roller type fixation device, however, these fixation devices do not press the toner image but only melt the toner image during the fixing process.
In the case of multicolor recording, it is necessary to smooth the surface of the fixed toner image. This is because, if the surface of the fixed toner image is uneven, a diffused reflection is generated on the surface whereby a fixed toner image having a high chroma is not obtainable or the color of the fixed toner image may vary in accordance with the angle of view. Since the heated roller type thermal fixation device melts the toner image under pressure as described above, it is possible to impart a favorable smoothness to the surface of the toner image compared with the flash or oven type fixation device, whereby the heated roller type is suitable for multicolor recording. In practice, however, even if the toner image is fixed by the heated roller type fixation device described above, it is difficult to impart sufficient surface smoothness to the fixed toner image. This is particularly true of a multicolor toner image having at least two colors. Specifically, since the toner itself consists of colored particles having a diameter of about 10 .mu.m, the surface of the recording paper becomes rough even if the toner image formed by such fine colored particles is transferred onto the recording paper and introduced into a nip between rigid surfaces of the heated roller and the backup roller. Therefore, it is impossible to impart a sufficient pressure onto the fixed toner image, which causes the surface of the fixed toner image to be unsatisfactory in smoothness, even by the above-mentioned heated roller type thermal fixation device. While the surface smoothness of the fixed toner image relies also on the manufacturing accuracy of the surfaces of the heated roller and the backup roller, the higher the accuracy of the surface smoothness, of the heated roller and the backup roller, the higher the manufacturing and/or maintenance cost.
Accordingly, to improve the surface smoothness of the fixed toner image, a heated roller type thermal fixation device has been proposed wherein an elastomeric covering such as a silicone rubber film is applied to either one or both of the heated roller and the backup roller. If the recording paper carrying the transferred toner image is introduced into a nip between the heated roller and the backup roller on which is applied the elastomeric covering, the elastomeric covering is deformed in accordance with the cross-sectional shape of the recording paper so that a sufficient pressure is applied to all the area of the recording paper. Thus, the surface smoothness of the fixed toner image is improved to a large extent. In this regard, since part of the toner image is liable to be transferred, i.e., offset-printed, onto the elastomeric covering on a heated roller with an elastomeric covering such as a silicone rubber film, an anti-offset liquid such as a silicone oil may be preferably coated on the surface of the elastomeric covering of the heated roller.
As stated above, while it is possible to improve the surface smoothness of the fixed toner image by applying the elastomeric covering onto the heated roller and the backup roller, there is another problem in that the elastomeric covering is liable to wear by abrasion. One of reasons for the abrasion of the elastomeric covering is that a front edge of the recording paper impinges onto the elastomeric covering to result in the cutting action on the latter when the recording paper is introduced into the nip between the heat roller and the backup roller. Also, the elastomeric covering may be subjected to the abrasion by a temperature sensor or a doctor blade for separating the recording paper. Particularly, the abrasion of the elastomeric covering in the fixation of a multicolor toner image is more significant than that in the fixation of a monochrome toner image. This is because the multicolor toner image is thicker than that of the monochrome toner image and therefore the pressure between the heat roller and the backup roller must be larger during the fixation of the multicolor toner image compared with that of the monochrome toner image. This causes a stepped portion in the elastomeric covering corresponding to a width of the recording paper when the recording paper passes through a nip between both the rollers, and this stepped portion is subjected to the cutting action due to the lateral edge of the recording paper. Of course, if the elastomeric covering is significantly worn, the surface smoothness of the fixed toner image is not sufficient. For example, the life of the heat roller with a silicone rubber covering is about 100,000 sheets in terms of the recording paper subjected to the fixation treatment. Particularly, in a case of a high-speed electrostatic recording apparatus wherein a large number of recording papers are treated, the hot roller type thermal fixation device thereof must be frequently replaced with a new device.